1. Reference to Related Patents.
Reference is made to U.S. Pat. No. 5,961,554, issued Oct. 5, 1999 to Frank S. Janson and Albert N. Santilli; U.S. Pat. No. 6,572,619, issued Jun. 3, 2003 to Albert N. Santilli; U.S. Pat. No. 6,572,654, issued Jun. 3, 2003 to Albert N. Santilli; and U.S. Pat. No. 6,673,075, issued Jan. 6, 2004 to Albert N. Santilli (collectively “the Porous Spacer Patents”), the disclosures of which are incorporated herein by reference.
2. Field of the Invention.
The invention relates to the arthrodesis of vertebral bodies and, more particularly, to an intervertebral spacer that will not migrate within its graft bed, thereby promoting rapid arthrodesis.
3. Description of the Prior Art.
Techniques and devices for fusing two or more vertebrae of the spine together are well known. Such techniques commonly are performed to correct problems, such as chronic back pain, which result from degenerated intervertebral discs. One technique for fusing together two or more vertebrae of the lumbar spine involves performing a discectomy, i.e., excising all or a portion of the disc between adjacent vertebrae, and inserting one or more portions of an intervertebral spacer of a desired shape between the adjacent vertebrae. The intervertebral spacer may be inserted by either an anterior or posterior approach to the spinal column depending on a number of factors, including the number of vertebrae to be fused and past operative procedures. Upon healing, the vertebrae desirably are fused together through the intervertebral spacer. Such surgical fusion is known as arthrodesis.
Intervertebral spacers have been described by a number of names, including spinal implants and spinal cages. For convenience, all such devices will be referred to herein as “intervertebral spacers” or simply “spacers.” There are a number of design features that must be taken into account in the construction of a suitable spacer. These features include the material from which the spacer is made, the external configuration of the spacer (and the mating graft bed in which the spacer will be implanted), and the extent and manner in which tissue ingrowth and bony fusion is permitted or promoted.
For many years, intervertebral spacers have been autogenic or autologous bone harvested from other areas of the body, such as the pelvis, allogenic bone taken from cadavers, or xenogenic bone, such as bovine bone sections. However, the use of substantially sized bone grafts can add complications to the fusion procedure. For example, when using an autologous bone graft, a second incision must be made in the patient to harvest the additional bone to be used in the graft, thus increasing the pain, blood loss, and trauma to the patient. When allogenic or xenogenic bone grafts are used there is a potential for the transmission of disease from the cadaver or other graft source to the patient.
More recently, non-biological spacers have been used, either alone or in combination with autologous, allogenic, or xenogenic fillers. A wide variety of materials have been used for these applications, but the best spacers have been made from metals that are bio-compatible with human tissue and that have desirable strength characteristics. Metals suitable for use as spacers include stainless steel as well as titanium, tantalum, niobium, and alloys and mixtures of these metals. Spacers made from these types of metals perform exceptionally well and, when properly sterilized, greatly minimize or eliminate the risk of infection or rejection.
As for the external configuration of the spacers, a variety of shapes, such as cylindrical or near-cylindrical, cubic, disc-like, and so forth has been employed. Reference is made to the Porous Spacer Patents for a disclosure of a number of shapes that can be employed for a spacer. Also, as shown in U.S. Pat. Nos. 4,878,915, 5,669,909 and 6,102,948, cylindrical and truncated conical spacers have been provided with threads along their length. These spacers can be threaded into a mating graft bed that is formed between adjacent vertebrae.
In considering whether a shape is suitable for a spacer, the difficulty in forming the graft bed must be taken into account. Generally, shapes are preferred that can be fitted into beds that can be prepared with minimal difficulty and with minimal removal of vertebral bone, provided that the shapes have adequate structural strength. Referring particularly to the cylindrical and truncated conical spacers referred to above, the graft bed usually is prepared by drilling. The bed can be tapped to receive the threads of the spacer, or the spacer can be forcibly threaded into the graft bed with the treads cutting their way through the end plates of the vertebrae or the remaining portions of the disc.
Another design consideration that must be taken into account is the extent to which tissue ingrowth and bony fusion is permitted or promoted. Some prior spacers have been provided in the form of bodies having pores or openings that permit bone ingrowth. Some of the openings are large enough to receive and hold bone or bone substitute. Since the object of the surgical procedure is to fuse the adjacent vertebrae, those spacers that promote bone ingrowth achieve a more rapid and stable arthrodesis.
Particularly effective non-biological spacers are disclosed in the Porous Spacer Patents. In these patents, spacers are made of a number of materials, including metal beads, metal wire mesh, or a combination of beads and wire mesh. Typically, the beads or wire mesh are made of a material such as titanium or titanium alloy. The spacers are made porous and strong by fusing the beads or wire mesh in a sintering operation that joins adjacent components at their points of contact. The spacers are completely porous throughout (on the order of 25 to 55 percent), which promotes desirable rapid tissue ingrowth and bony fusion. In addition to their inherent porosity, the spacers also can be provided with openings that can receive and hold bone or bone substitute.
In order to install the spacers disclosed in the Porous Spacer Patents, a graft bed is prepared in the spinal column by excising defective portions of a disc and portions of the adjacent vertebral bodies. If, for example, a cubic spacer is to be implanted, a cubic graft bed having approximately the same dimensions as that of the spacer would be prepared. Unfortunately, it is difficult and time-consuming to create a graft bed during the course of a surgical procedure that precisely conforms to the size and shape of a pre-existing spacer whose dimensions cannot be changed. Accordingly, it is possible for the spacer to fit relatively loosely in the graft bed. Such looseness may permit the spacer to change position, or migrate, while the healing process occurs. Since it takes about two months or more after implantation of a spacer for sufficient tissue ingrowth and bony fusion to occur, any post-operative migration of the spacer could extend the healing process or possibly render the spacer ineffective for its intended purpose.
The problem of migration also exists with respect to cylindrical or truncated conical spacers. Due to their threaded nature, such spacers can “back out” or otherwise become loosened in the graft bed after implantation. While the use of such spacers is desirable because it is relatively easy to form a graft bed by drilling, post-operative migration remains a problem.
Certain spacers have been proposed that include a plurality of radially projecting fins included as an integral part of a housing or as a movable assembly disposed within a housing. The housings typically are in the form of modified rectangular cubes that are placed in a graft bed. The housings or the movable assemblies can be rotated such that the fins cut into the vertebral end plates and thereby lock or attempt to lock the spacers in place. Such spacers are disclosed in the following U.S. Pat. Nos.: 6,537,320; 6,767,367; 6,923,830; 6,981,975; 7,056,342; and 7,112,206. While the referenced patents attempt to address the problem of spacer migration, it is believed that the shape of the spacers and the configuration of the fins are not an optimum approach to solving the problem.
In addition to post-operative migration, a number of difficulties still remain with many of the spacers currently available. While it is recognized that a hollow spacer containing bone or bone substitute disposed within the implant is a desirable technique for achieving fusion, some of the prior art devices have difficulty in achieving this fusion, at least without the aid of some additional stabilizing device, such a rod or plate. It has been found that the size of the openings in the spacer plays an important role in avoiding stress shielding of any bone implanted within the spacer. In other words, if the openings are too small or improperly configured, autologous bone will not experience the compression loading that typically is found to be necessary to ensure rapid and complete fusion. In this instance, the bone impacted within the spacer may resorb or evolve into simply fibrous tissue, rather than the desired bony fusion mass. On the other hand, the bone ingrowth openings must not be so large or extensive in number that the spacer does not have enough structural integrity to support the heavy load and bending moments that will be applied to it.
Desirably, a strong, bio-compatible intervertebral spacer would exist that could be fitted into an easily prepared graft bed. Any such spacer hopefully would be easy to install and would be tightly secured in place once installed so that any post-operative migration of the spacer, with consequent disruption of the arthrodesis process, would be prevented. Preferably, any such spacer would permit rapid tissue ingrowth and bony fusion while exhibiting excellent strength characteristics.